Steam now offers the promised free release of an updated version of Alien Swarm, which includes the complete code base for the game. They say: "Available free of charge, the game thrusts players into an epic bug hunt featuring a unique blend of co-op play and squad-level tactics. Along with the game get the complete code base for Alien Swarm that features updates to the Source engine as well as the SDK."

I've almost got it downloaded, though Steam is telling me it's slightly over 2GB. It took me 20 minutes just to get it started though, and the download rate has been all over the place (steady at 850 KB/s right now). I hope to jump into it a little later, if the Steam servers haven't completely died by then.